1. Field of the Invention
The present invention relates to a steering system for a motor vehicle, and more particularly to a four-wheel steering system for a motor vehicle such as an automobile.
2. Description of Relevant Art
There are known various four-wheel steering systems for steering the front and rear wheels of motor vehicles.
Basically, in many four-wheel steering systems, the ratio (steering angle ratio) between a front wheel steering angle, i.e., the angle through which the front wheels are to be steered, and a rear wheel steering angle, i.e., the angle through which the rear wheels are to be steered, varies depending on the speed of travel of the motor vehicle, as disclosed in U.S. Pat. Nos. 4,703,822 and 4,730,839, for example. More specifically, these four-wheel steering systems make the steering angle ratio negative, i.e., steer the rear wheels in a direction opposite to the direction in which the front wheels are steered, in a low speed range, and make the steering angle ratio positive, i.e., steering the rear wheels in the same direction as the front wheels, in medium and high speed ranges. In the low speed range, since the turning behavior of the motor vehicle is mainly governed by the yawing acceleration, the motor vehicle can turn quite well as desired. In the medium and high speed ranges the motor vehicle can change lanes quickly because the turning behavior thereof is mainly governed by the lateral acceleration, which is greater as the motor vehicle runs faster.
Japanese Laid-Open Patent Publication No. 59-77968 published on May 4, 1984 discloses a four-wheel steering system which basically keeps the steering angle ratio positive. Specifically, the positive steering angle ratio increases as the motor vehicle speed increases. When the front wheel steering angle becomes greater than a predetermined value at each motor vehicle speed, the rear wheel steering angle is maintained at the predetermined value. The rear wheel steering angle maintained at the predetermined value is therefore an upper limit steering angle for the rear wheels at each motor vehicle speed. The predetermined value is selected such that it is smaller as the motor vehicle speed is lower. With the disclosed four-wheel steering system, inasmuch as the steering angle ratio increases as the motor vehicle speed goes higher, the turning response of the motor vehicle is improved when it changes lanes at medium and high speeds. The maneuverability of the motor vehicle is improved since the upper limit steering angle for the rear wheels increases as the motor vehicle speed increases.
Experiments conducted by the applicant have shown that the motor vehicle should turn well while the steering wheel angle is small in medium and high speed ranges, i.e., while the steering wheel is in the vicinity of its neutral position and the front wheel steering angle is small. Stated otherwise, during an initial phase of the steering action, the driver should preferably feel the yawing acceleration.
According to the four-wheel steering systems disclosed in the above two U.S. Patents, the rear wheels are steered in the same direction as the front wheels in the medium and high speed ranges. Therefore, the motor vehicle with these disclosed four-wheel steering systems cannot turn well in the medium and high speed ranges, irrespective of the magnitude of the steering wheel angle.
According to the four-wheel steering system disclosed in the above Japanese Laid-Open Patent Publication, when the front wheel steering angle exceeds the predetermined value in all of the vehicle speed ranges, the rear wheel steering angle, which is in the same direction as the front wheel steering angle, is kept at its upper limit that depends on the motor vehicle speed. Insofar as the front wheel steering angle is large, the turning motions of the motor vehicle are governed by the yawing acceleration, allowing the motor vehicle to turn well, regardless of the motor vehicle speed. However, while the front wheel steering angle is small, the motor vehicle cannot turn well regardless of the motor vehicle speed, because the rear wheels are turned in the same direction as the front wheels as described above.
The present invention has been made in an effort to improve the conventional four-wheel steering systems for motor vehicles.